New maps from orbiting sensors that can detect flashes
of lightning even during the daytime reveal where on Earth the
powerful bolts will most likely strike.

Dec. 5, 2001: Lightning.
It avoids the ocean, but likes Florida. It's attracted to the
Himalayas and even more so to central Africa. And lightning almost
never strikes the north or south poles.

These are just a few of the things NASA scientists have learned
using satellites to monitor worldwide lightning.

"For the first time, we've been able to map the global
distribution of lightning, noting its variation as a function
of latitude, longitude and time of year," says Hugh Christian,
project leader for the National Space Science and Technology
Center's (NSSTC's) lightning team at NASA's Marshall Space Flight
Center.

Left: A lightning bolt strikes
the Atlantic Ocean near Florida. According to a new NASA map
of global lightning rates, such strikes over open ocean waters
are rare. Image courtesy NOAA.

This new perspective on lightning is possible thanks
to two satellite-based detectors: the Optical Transient Detector
(OTD) and the Lightning Imaging Sensor (LIS). "The OTD and
the LIS are two optical sensors that we've flown in lower Earth
orbit," says Christian, whose team developed the sensors.
"The OTD was launched in 1995 and we got five good years
out of it. The LIS was launched on the Tropical Rainfall Measuring
Mission satellite in 1997 and it's still going strong."

"Basically, these optical sensors use high-speed cameras
to look for changes in the tops of clouds, changes your eyes
can't see," he explains. By analyzing a narrow wavelength
band around 777 nanometers -- which is in the near-infrared region
of the spectrum -- they can spot brief lightning flashes even
under daytime conditions.

Before OTD and LIS, global lightning patterns were known only
approximately. Ground-based lightning detectors employing radio-frequency
sensors provide high-quality local measurements. But because
such sensors have a limited range, oceans and low-population
areas had been poorly sampled. The development of space-based
optical detectors was a major advance, giving researchers their
first complete picture of planet-wide lightning activity.

The new maps show that Florida, for example, is one place
where the rate of strikes is unusually high. Dennis Boccippio,
an atmospheric scientist with the NSSTC lightning team, explains
why: "Florida experiences two sea breezes: one from the
east coast and one from the west coast." The "push"
between these two breezes forces ground air upward and triggers
thunderstorms.

Within thunderclouds, turbulence spawned by updrafts causes
tiny ice crystals and water droplets (called "hydrometeors")
to bump around and collide. For reasons not fully understood,
positive electric charge accumulates on smaller particles --
that is, on hydrometeors smaller than about 100 micrometers --
while negative charges grow on the larger ones. Winds and gravity
separate the charged hydrometeors and produce an enormous electrical
potential within the storm.

"Lightning is one of the mechanisms
to relax this build-up," says Boccippio.

Right:
Lightning
is a sudden discharge of electricity between charged regions
of thunderclouds and the ground. Only about 25 percent of lightning
strikes are cloud-to-ground. The rest are either cloud-to-cloud
or intracloud. [more
information]

Another lightning hot spot is in the Himalayas where the extreme
local topography forces the convergence of air masses from the
Indian Ocean.

And where does lightning strike most frequently? Central Africa.
"There you get thunderstorms all year 'round," Christian
says. "[It's a result of] weather patterns, air flow from
the Atlantic Ocean, and enhancement by mountainous areas."

The satellite data also track patterns of lightning intensity
over time. In the northern hemisphere, for example, most lightning
happens during the summer months. But in equatorial regions,
lightning appears more often during the fall and spring.

This seasonal variation contributes to a curious north-south
asymmetry: Lightning ignites many of North America's late-summer
wildfires, while some studies find that wildfires in South America
are sparked more often by humans. Why the difference? It's simply
because lightning in South America happens during a season when
the ground is damp. Summertime lightning bolts in North America
strike when the ground is dry and littered with fuel for fires.

Meanwhile,
areas such as the Arctic and Antarctic have very few thunderstorms
and, therefore, almost no lightning at all.

"Oceanic areas also experience [a dearth of lightning],"
Christian says. "People living on some of the islands in
the Pacific don't describe much lightning in their language."
The ocean surface doesn't warm up as much as land does during
the day because of water's higher heat capacity. Heating of low-lying
air is crucial for storm formation, so the oceans don't experience
as many thunderstorms.

Left: Relax. Lightning rarely strikes Pacific Islands.

According to Boccippio these global patterns probably aren't
much influenced by human activity. Some people have suggested
that buildings and metal communications towers increase the overall
frequency of lightning strikes. But, "lightning that does
make it to the ground is pretty much creating its own channels,"
Boccippio says. "The likelihood that we are changing the
amount of cloud-to-ground strikes with construction of towers
is very slim." He cautions, however, that this has not been
verified experimentally.

To answer such questions, a new lightning detector -- the
Lightning Mapper Sensor or "LMS" -- is on the drawing
board at the NSSTC. The proposed instrument would circle our
planet in a geostationary orbit over the United States, detecting
all forms of lightning with a high spatial resolution and detection
efficiency.

Right: The Lightning Imaging
Sensor (LIS)
on board TRMM monitors lightning flashes on the Earth below by
collecting 500 images per second. The same optical technology
will likely be found in future space-based lightning sensors.
Image courtesy NSSTC.

The LMS or something like it could provide valuable -- even
life-saving -- data to weather forecasters. "The same updrafts
that drive severe weather often cause a spike in the lightning
rate [at the onset of] a storm," explains Boccippio. So,
measuring the rate of lightning flashes in real time might offer
a way to identify potentially deadly storms before they
become deadly.

Clearly, lightning research is a field truly crackling with
potential. You can learn more about it from the Global Hydrology
and Climate Center's web site: Lightning
and Atmospheric Electricity.

Learning from Lightning -- (Science@NASA) Little by
little, lightning sensors in space are revealing the inner workings
of severe storms. Scientists hope to use the technique to improve
forecasts of deadly weather.